Microwave oven



w. N. PARKER 3,422,240

MICROWAVE OVEN Jan. 14, 1969 Filed Nov. 18, 1965 6- ba 6 8 4 z 6 4 I 0 7w l 8 0 M 4 unm d ll 3% 2 a 6 i i 5/ 6 U U l lllll llll l I N VEN TOR. ZU/zz mm M 2404 5 7M% 53;

firm/way United States Patent 3,422,240 MICROWAVE OVEN William N. Parker, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware Filed Nov. 18, 1965, Ser. No. 508,535 US. Cl. 219-1055 8 Claims Int. Cl. H! 9/00; H05!) 9/06 ABSTRACT OF THE DISCLOSURE Disclosed is a configuration for a microwave cavity wherein dielectric articles may be heated. The walls of the cavity are formed of an electrically conductive material which in turn provide a resonant chamber. Electrically conductive supporting means, insulated from the walls of the cavity, are positioned therein. Means are provided for conductively coupling microwave energy to the walls of the cavity and to the conductive supporting means. The resulting electric field developed between the walls and the supporting means evenly heats the articles placed upon the latter.

This invention relates generally to microwave heating apparatus and more particularly to a new and improved oven to provide for the uniform heating of lossy dielectric articles therein.

It is known to provide relatively fast heating of lossy dielectric articles by the use of high frequency energy in the microwave range. While microwave ovens have been made commercially available in the past, their acceptance has been impeded for several different reasons.

One factor which has prevented the wide acceptance of prior art microwave ovens is the difficulty of obtaining proper energy distribution within the microwave oven so as to provide uniform heating and cooking of the lossy dielectric articles therein. Due to the characteristics of microwave energy, it has been found in the cooking of food articles, for example, that certain portions of the food articles tend to be heated at a much higher rate than the other portions, with the result that one portion is undercooked when another portion is properly cooked. In many cases, some portions must be overcooked to obtain proper cooking of the low heating portions.

Numerous hot and cold spots may occur almost anyplace with the known microwave ovens, depending on the particular mode. In the past, various means have been proposed for evenly distributing microwave power in an empty cavity, such as reciprocating reflector plates, paddle wheel stirrers, rotating food support platforms, but to date no truly satisfactory solution has been found to evenly distribute microwave energy with-in a lossy dielectric positioned for heating within the cavity.

Accordingly, it is a general object of this invention to provide an improved microwave heating apparatus for achieving more uniform heating of lossy dielectric articles within an oven.

It is another object to provide new and improved microwave cooking apparatus for dielectric articles, the apparatus having unique means for concentrating the microwave energy at a desired portion of the dielectric to evenly distribute rnicrowave power throughout the dielectric and thereby obtain uniform cooking of the dielectric, for example, food articles.

The present invention concerns itself with an oven of the type using microwave energy to heat dielectric articles. The walls of the oven, which form a resonant chamber, are made of an electrically conductive material. A means for supporting the article or articles to be heated is positioned within the cavity. The supporting means, which is made of an electrically conductive material, is insulated "ice from the chamber walls. Means are provided to couple microwave energy to the walls and to the supporting means. The resulting electric field developed between the walls and the supporting means evenly heats the articles placed on the supporting means.

This invention will best be understood from the following detailed description when read in connection with the accompanying drawing in which:

FIG. 1 is a sectional view of an oven constructed in accordance with one embodiment of the invention;

FIG. 2 is a diagram showing the curernt flow and electrical field configuration at an instant in time within the oven shown in FIG. 1; and

FIG. 3 is a circuit diagram of the voltage and current relationships of FIG 2.

The oven 4 shown in FIG. 1, comprises a cavity 6 formed by a substantially flat base 8 and an upstanding shell 10 provided with a substantially flat top wall 12. The shell 10, bottom and top walls 8 and 12 are made of an electrically conductive material, such as silver coated brass or copper. The walls (8, 10 and 12) of the cavity 6 are preferably arranged so as to be symmetrical about a vertical axis. For example, the shell 10 can take the for-m of a cylinder or consist of four electrically connected rectangular sides. The cross sectional shape of the oven 6 is not visible in FIG. 1. The top wall 12 has a hole 14 which is preferably symmetrically located about the vertical axis of the cavity 6. A cylindrically shaped plunger 16 is inserted axially through the hole 14 in the top wall 12. The plunger 16 can be made of silver plated brass or other good electrical conductor. Electrical contact is made between the top wall 12 and the plunger 16 by means of metal flexible fingers 18 aflixed to the inner surface 52 of the top wall 12 within the cavity 6. The plunger 16 is adjustably held in position by any desired means such as a ring stand or other clamping device (not shown).

The bottom wall 8 has a hole 20 therein which is preferably symmetrically positioned about the vertical axis of the cavity 6. Affixed to the exterior surface 22 about the hole 20 in the bottom wall -8 is the outer conductor 24 of an L-shaped coaxial line 26. The hollow, cylindrically shaped outer conductor 24, which is made of a rigid electrical material such as copper, terminates in a laterally extending flange 28. The flange 28 is employed to secure the outer conductor 24 of coaxial line 26 to the cavity 6. The inner conductor 30 of the coaxial line 26, is an L-shaped hollow member which is coaxial within the outer conductor 24. Small holes 32 and 34 are formed respectively in the lateral sections 36 and 38 of the outer and inner conductors 24 and 3 0. The holes 32 and 34 are preferably symmetrically located about the axis of the upright shell 10 of the cavity 6. A rod-shaped conductive member 40 is affixed to a rod-shaped insulator 42. The insulator rod 42 can be made of any rigid insulation, such as Teflon. The two members 40 and 42 can be aifixed in a known manner as by threading one Within the other. The two members 40 and 42 are arranged axially Within the hollow inner conductor 30 of the coaxial line 26. The electrically conductive member 40 extends through the hole 20 and into the cavity 6. The insulator rod 42 extends without the hole 32 in the outer conductor 24. The inner conductor 30 terminates at a point below the bottom wall 8 of the oven 4. Resilient metal fingers 44 are affixed to the inner conductor 30 to insure electrical contact between it and the conductive member 40. The two members 40 and 42 are so arranged as to permit the conductive member 40 to telescope within the inner conductor 30.

A platform 46 within the cavity is aflixed to the conductive member 40 as by soldering or welding. Tlhe platform 46 is made of an electrically conductive substance, such as aluminum. The platform 46 has two substantially planar surfaces 48 and 50 parallel to the interior surfaces 52 and 54 of the top and bottom walls '12 and 8. The peripheral edge 56 of the platform 46 is parallel to the shape of the interior surface 58 and is preferably symmetrical about the vertical axis of the upright shell 10. The platform 46 may be raised and lowered by adjustably moving the insulating member 42. For ideal electrical performance the platform 46 should be only a little smaller than the inside dimensions of the cavity 6.

Microwave energy is provided to the oven 4 by means of a source (not shown) coupled to the coaxial line 26. Dielectric substances 60, such as food, to be heated are placed on the platform 46 symmetrically about the vertical axis of the upright shell as nearly as practical. The fod or other substances 60 to be heated can be placed in the oven 4 through a suitably constructed door (not shown) in the shell 10 of the oven 4.

The operation of the oven can best be understood by reference to FIGS. 2 and 3 taken in conjunction with FIG. 1. Microwave energy is coupled to the walls 8, 10 and 12 of the cavity 6 and the platform 46 by means of the coaxial line 26. The microwave energy will resonate within the cavity 6 if it is properly tuned by the Plunger 16. Currents I shown in FIGS. 2 and 3 at a given instant in time will flow in the interior surfaces of the walls of the oven 4 in a direction opposite the direction of current flowing in the platform 46. An electric field E is established between the walls of the cavity 6 and the surfaces of the platform 46. The mode or shape of the field established will conform to the structure of the tuned cavity 6 and the microwave energy supplied thereto.

The cavity 6 functions electrically much like a tuned resonant circuit as shown in FIG. 3. There is indicated in FIG. 3 an auto transformer L as part of the parallel resonant circuit. This transformer L is a representation of the coupling of microwave energy to the cavity 6 by means of the platform 46. In FIG. 3 the transformer turns are indicated by the letters d and d Correspondingly in FIG. 2 al and d are the relative distances of the platform 46 between the top and bottom surfaces 52 and 54 of the cavity 6. The high electric field E between the top surface 48 of the platform 46 and the inner surface 52 of the top wall 12 represents the high electric field E found between the plates of a capacitor (C in FIG. 3) of a parallel resonant circuit.

At resonance the impedance of a parallel resonant circuit approaches infinity. Likewise, the resonant impedance of cavity 6 is very high in the absence of dielectric 60. Consequently, in order to couple a maximum amount of power from the microwave line 26 to the empty cavity 6 it is necessary to make d very small with respect to d However, when cavity 6 contains lossy dielectric 60, the impedance is much smaller. Therefore, d is made larger to compensate for the presence of dielectric 60.

The area of maximum electric field E is between the top surface 48 of the platform 46 and the inner surface 52 of the top wall 12 and particularly over the center of the platform 46. Food 60 is placed on the platform 46 symmetrically about the vertical axis where it will be subjected to high intensity heat. Because the food 60 is proximate to the source of microwave energy within the cavity 6, it will heat symmetrically and uniformly over a substantial region. Previously made microwave ovens required that substances to be heated be placed so that different portions were at various distance from the source of microwave energy causing uneven heating.

The preferred form of cavity for the present invention is one that has a cylindrical shell and a platform that is symmetrically placed with respect to the vertical axis of the shell 10 and has a flat, circular top surface 48. However, cavities having other shapes can be used. As an example, a cavity wherein the top and bottom walls 12 and 8 are square can be used to heat food and has been constructed. The top and bottom walls 12 and 8 were 8.75 by 8.75 inches. The shell 10 had four rectangular sides, each side surface being 8.75 by 8.75 inches high. The platform 46 had in the given application a top surface 48 which measured 5 by 5 inches and had an edge surface 56 which was .125 inch thick. The platform 46 was joined to coaxial line 26 having a characteristic impedance of 50 ohms. Microwave energy of 915 mc, was supplied to the coaxial line 26. The cylindrical plunger 16 had a diameter of three inches, and extended into the cavity 6 so that the cavityresonated at 915 mc. The electric field E propagated within the cavity 6 in the TE mode. Assuming that the dielectric substance 60 takes the form of hamburger, for example, and the microwave energy is supplied at about 1,500 watts to an oven constructed as just described, the hamburger will be uniformly cooked in approximately 15 seconds. The hamburger 60 is merely placed at the center of the platform 46 and the platform 46 is adjusted so that the height d from the bottom surface 54 of the cavity 6 is in the example given approximately .25 inch. In the same way uniform heating of variously shaped dielectrics may be readily accomplished in accordance with the particular application.

What is claimed is: 1. A microwave oven of the type comprised of a resonant chamber, said oven comprising:

an electrically conductive member for supporting substances to be heated within said chamber, said member having a perimeter slightly smaller than the inner perimeter of said chamber, said member being electrically insulated from said chamber, and means for applying microwave energy conductively coupled to said member to establish an electric field between said member and said chamber to heat said substance. 2. A microwave oven for heating substances comprismg:

an enclosed resonant chamber having walls of electrically conductive material, electrically conductive means insulated from said chamber for supporting within said chamber said substances, said supporting means having a perimeter slightly smaller than the inner perimeter of said chamber, and means for conductively coupling microwave energy to said Walls of said chamber and to said supporting means for establishing an electric field therebetween for heating said substances. 3. A microwave oven for heating substances comprismg:

an enclosed resonant chamber having walls of electrically conductive material, electrically conductive means insulated from said chamber for supporting within said chamber said substances, said supporting means having a perimeter slightly smaller than the inner perimeter of said chamber, means for conductively coupling microwave energy to said walls of said chamber and to said supporting means for establishing an electric field therebetween for heating said substances, and means for tuning said chamber to be resonant at the frequency of said microwave energy. 4. A microwave oven for heating substances comprisan enclosed resonant chamber having walls of electrically conductive material, electrically conductive means for supporting within said chamber said substances, said supporting means having a perimeter slightly smaller than the inner perimeter of said chamber, means for conductively coupling microwave energy to said walls of said chamber and to said supporting means, means for adjusting said supporting means to match the impedance of said microwave energy coupling means to the impedance of said chamber, and

means for tuning said chamber to be resonant at the frequency of said microwave energy.

5. In combination:

an enclosed resonant chamber having walls of electrically conductive material,

electrically conductive means for supporting within said chamber substances to be heated, said supporting means having a perimeter slightly smaller than the inner perimeter of said chamber,

said chamber being axially symmetrical and said sup porting means being centrally located and symmetrically about said axis,

means for conductively coupling microwave energy to said walls of said chamber and to said supporting means,

means for adjusting said supporting means to match the impedance of said microwave energy coupling means to the impedance of said chamber, and

means for tuning said chamber to be resonant at the frequency of said microwave energy.

6. In combination:

a flat bottom Wall of electrically conductive material,

an upstanding shell of electrically conductive material conductively afiixed to said bottom wall,

a flat top wall of electrically conductive material conductively afllxed to said shell, said bottom wall together with said shell and said top wall forming an enclosed resonant chamber, said chamber being symmetrical about a vertical axis,

a flat supporting member of electrically conductive material positioned within said chamber, said supporting member having a surface substantially parallel to said top wall and symmetrical about said vertical axis and electrically insulated from said walls to said chamber, said member having a perimeter slightly smaller than the inner perimeter of said chamber,

a coaxial line having an inner and outer conductor, said outer conductor being electrically connected to said walls of said chamber and said inner conductor being connected to said supporting member,

means for tuning said chamber to resonate at a particular frequency, and

means for adjustably positioning said supporting member within said chamber to match said coaxial line to said resonant chamber.

7. A microwave oven for heating a dielectric substance comprising a flat, square bottom wall of electrically conductive material,

a rectangular upstanding shell of electrically conductive material aflixed to said bottom wall,

a flat square top wall of electrically conductive material affixed to said shell, said bottom wall together with said shell and said top wall forming an enclosed resonant chamber, said chamber being symmetrical about a vertical axis,

a fiat square member of electrically conductive material for supporting said substance positioned within said chamber, said supporting member having a surface substantially parallel to said top wall and symmetrical about said vertical axis and electrically insulated from said chamber, said member having a perimeter slightly smaller than the inner perimeter of said chamber,

a coaxial line having an inner and outer conductor,

.means to connect said outer conductor to said walls of said chamber,

means to connect said inner conductor to said supporting member,

said coaxial line serving to conductively couple microwave energy to said walls and to said supporting member to establish an electric field therebetween for heating said substance,

means for tuning said chamber to be resonant at the frequency of said microwave energy, and

means for adjustably positioning said supporting member to match the impedance of said coaxial line to that resonant chamber.

8. An oven for heating a dielectric substance therein,

comprising:

a flat circular bottom wall of electrically conductive material,

a cylindrical upstanding shell of electrically conductive material affixed to said bottom wall,

a flat circular top wall of electrically conductive material aflixed to said shell, said bottom wall together with said shell and said top wall forming an enclosed resonant chamber, said chamber being symmetrical about a vertical axis,

a flat circular member of electrically conductive material for supporting said substance positioned within said chamber, said supporting member having a surface substantially parallel to said top wall and symmetrical about said vertical axis and electrically insulated from said chamber, said member having a perimeter slightly smaller than the inner perimeter of said chamber,

a coaxial line having an inner and outer conductor, said outer conductor being electrically connected to said walls of said chamber and said inner conductor being electrically connected to said supporting member,

said coaxial line serving to conductively couple microwave energy to said walls and to said supporting member to establish an electric field therebetween for heating said substance,

means for tuning said chamber to be resonant at the frequency of said microwave energy, and

means for adjustably positioning said supporting member to match said coaxial line to said resonant chamber.

References Cited UNITED STATES PATENTS 3,172,987 3/1965 Fitzmayer 219-1055 3,294,879 12/1966 Jacobs 219-1055 X 3,300,615 1/1967 Smith 21910.55

3,321,605 5/1967 Reker 219--10.55

FOREIGN PATENTS 1,444,384 5/ 1966 France.

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner. 

